1. Field of the Invention
The present invention generally relates to color document scanning systems and more particularly relates to a full width image sensor comprising multiple arrays of photodetectors laid out along the moving direction of a document, wherein the multiple arrays of photodetectors are sequentially exposed to the document to produce scanning signals of high fidelity and strength.
2. Description of the Related Art
There are many applications that need optical scanners to convert paper-based objects, such as texts and graphics, to an electronic format that can be subsequently analyzed, distributed and archived. One of the most popular optical scanners is flatbed scanners that convert scanning objects, including pictures and papers, to images that can be used, for example, for building World Wide Web pages and optical character recognition. Another popular optical scanner is what is called sheet-fed scanners that are small and unobtrusive enough to sit between a keyboard and a computer monitor, or can be integrated into a keyboard or a PDA to provide a handy scanning means. Most optical scanners are referred to as image scanners as the output thereof is generally in digital image format.
Structurally, an optical scanner generally comprises a photo-sensing module that converts a document image optically into its corresponding electronic signal. Typically, a photo-sensing module comprises an illumination system, an optical system, an image sensor and an output circuit. The illumination system is used to illuminate the document image being scanned. The optical system is used to direct and focus the image light reflected from the document image onto the image sensor. Physically, the image sensor comprises a plurality of photodiodes, photo-transistors or Charge Coupled Devices (CCD), referred to as photodetectors hereafter, that are sensitive to an incident light and produces an electronic signal, called the pixel or charge signal of each photodetector. Generally, the pixel signal is proportional to the intensity of the incident light, and the brighter the incident light is, the stronger the pixel signal will be. The output circuit is used to amplify if necessary and convert these pixel signals into an appropriate digital image format for further processing.
The image sensor and its associated signal processing circuitry are generally fabricated with Complementary Metal-Oxide Semiconductor (CMOS) or CCD semiconductor wafer processing technology and configured in either a one-dimensional or two-dimensional array. Those one-dimensional arrays are typically employed in the optical scanners including photocopy and facsimile machines while the two-dimensional arrays are typically used in instant image capturing devices (i.e. digital cameras and camcorders).
The operation of an image sensor comprises two processes, the first being the light integration process and the second being the signal readout process. During the light integration process, each photodetector captures the incident photons of the reflected light from a document that is being imaged or scanned and converts the total number of the incident photons into a proportional amount of an electronic charge or an equivalent pixel signal. At the end of the light integration process, the photodetector is masked so that no further photons would be captured. Next, the photodetector starts the signal readout process during which the pixel signal in the subject photodetector element is read out, via a readout circuit, to a data bus or video bus.
To save cost, many flatbed and sheet-fed scanners employ an image sensor of one-dimensional array. As the document image to be scanned is two-dimensional, relative and line-by-line movement between the document and the image sensor with concurrent image capturing by the image sensor is required in order to capture an image of the complete document. Additionally, in the case of scanners that are capable of capturing the information off a color document, there are generally three sets of distinctly different component color information to be captured for a color image.
In addition to the need of a high degree of fidelity of the captured image, another important requirement of an image scanner is its speed, or scanning throughput. This is the time it takes for a scanner to scan an entire document page. The shorter the capture time is, the more productive the scanner is, hence the more desirable it is to users.
It is generally understood that the word xe2x80x9clinexe2x80x9d within the context of xe2x80x9cline-by-line movementxe2x80x9d refers to a pixel line, a scanning line position or simply a scanning line corresponding to the resolution of the scanner which is typically expressed in terms of lines, pixels, or per inch. Naturally, the higher the resolution of the scanner is, the better the fidelity of the captured image will be. For a typical image scanner, a minimum resolution of 300 Pixels Per Inch (PPI) is expected with 600 PPI demanded in many cases. Thus, to scan a typical page of 8.5xe2x80x3xc3x9711xe2x80x3 letter sized document at a resolution of 300 PPI the following total number of scanning lines must be imaged:
11 inchxc3x97300 lines/inch=3300 lines.
In a typical photocopy (Xerox) machine, scanning these lines is just a matter of seconds because the photocopy machine can provide a very strong illumination source so that the integration time for photodetectors is very short. Given the fact that the integration time is typically long for the photodetectors in a small optical scanner as the scanner could not provide a strong illumination source, having to scan such a large number of scanning lines per page would take a noticeably long time.
Therefore, it is desirable for an image sensor that, when employed in a scanner, could provide a high scanning speed without requiring additional stronger illumination.
The present invention has particular applications to scanners such as desktop, sheet-fed scanners and facsimile machines. According to one aspect of the present invention, an image sensor employing multiple arrays of photodetectors is disclosed. According to one aspect of the present invention, every two of three arrays are shielded with a non-transparent material. When charge signals are generated in the non-shielded array in response to light reflected from a scanning document illuminated by one of three colored lights, the charge signals are shifted to a next adjacent shielded array so that another set of charge signals can be generated under another one of the three colored lights. Essentially, the shielded arrays are used as buffers to separate the charge signals independently generated under three different colored lights. Subsequently, the charge signals corresponding to the same colored light can be accumulated to produce a much stronger scanning signal.
The image sensor can be advantageously used in a portable scanner for scanning a document. Typically, a portable scanner is equipped with a low illumination source. With the image sensor of the current invention, the scanning throughput can be substantially increased and at the same time, the image quality is of great improvement.
Accordingly, it is an object of the present invention to provide an image sensor that output scanning signals of high fidelity and low noises.
Other objects, together with the foregoing are attained in the exercise of the invention in the following description and resulting in the embodiment illustrated in the accompanying drawings.